Monolayer-capped metallic nanoparticles (MCNPs) have attracted significant attention during the past two decades, due to their unique bulk and surface properties (Mokari, Nano Rev. 2011, 2, 5983; Tisch & Haick, MRS Bull. 2010, 35(1), 797; Tisch & Haick, Rev. Chem. Eng. 2010, 26, 171; Haick, J. Phys. D 2007, 40, 7173; Daniel & Astruc. Chem. Rev. 2004, 104, 293). Sensors based on MCNP films are of special interest, mainly due to their controllable selectivity, high sensitivity, low detection limits, fast response and recovery times, small size, low-output impedance and easy integration in standard microelectronic devices (Joseph et al., J. Phys. Chem. C 2008, 112, 12507; Joseph et al., Sens. Actuat. B 2004, 98, 188; Herrmann et al., Phys. Rev. B 2007, 76, 212201; Wang et al., Langmuir 2010, 26, 618; Dovgolevsky et al., Small 2009, 5, 1158). These features allowed the successful implementation of MCNPs in a wide variety of applications, varying from detection of organic compounds in aqueous solutions (Raguse et al., J. Phys. Chem. C. 2009, 113, 15390; Chow et al., Sens. Actual. B 2009, 143, 704; Cooper et al., J. Anal. Chem. 2010, 82, 3788) to detection of trace analytes in the gas phase (Joseph et at, Sens. Actuat. B 2004, 98, 188; Joseph et al., J. Phys. Chem. B 2003, 107, 7406; Dovgolevsky et al., J. Phys. Chem. C 2010, 114, 14042), and even diagnosis of diseases from breath samples (Tisch & Haick, Rev. Chem. Eng. 2010, 26, 171; Peng et al., Nature Nanotechnol. 2009, 4, 669; Peng et al., Br. J. Cancer 2010, 103, 542; Barash et at, Small 2009, 5, 2618; Hakim et al., Br. J. Cancer 2011, 104, 1649; Marom et al., Nanomed. (Future Medicine) 2012, 7, 639; Shuster et al., Breast. Cancer Res. Treat. 2011, 126, 791) or in vitro samples (Barash et al., Small 2009, 5, 2618; Barash et al., Nanomed, Nanotech. Bio. Med. 2012, 8, 580). For use in chemiresistors, the metal cores, consisting either of a single metal or of an alloy of two or more metals, provide the electronic conductivity. The non-conductive organic matrix that coats the nanoparticles provides adsorption sites for the analyte molecules. The combination of metal cores and capping organic matrix provides two counteracting effects upon analyte adsorption: (i) three-dimensional swelling of the MNCP film that increases the interparticle tunneling distance for charge carriers and, hence, the film's resistance; and (ii) increasing the permittivity of the organic matrix around the metal cores that decreases the potential barriers between the metal cores, and, hence, the film's resistance (Haick, J. Phys. D 2007, 40, 7173). To tune the sensing signals of the MCNPs, a popular approach has been applied, namely to use derivatives of different backbones (Wang et al., J. Mater. Chem. 2007, 17, 457; Rowe et al., Chem. Mater. 2004, 16, 3513) and/or different electron-withdrawing or electron-accepting functional groups (Cooper et al., J. Anal. Chem. 2010, 82, 3788; Joseph et al., J. Phys. Chem. C. 2007, 111, 12855). This approach, however, incorporates synthesis challenges either of the molecular ligands or the MCNPs per se. In addition, different ligands usually result in different steric hindrance between the adjacent ligands adsorbed on the NP surface, different molecular densities on the NP surface, and therefore, different NP sizes and/or NP size distribution (Wang et al., J. Mater, Chem. 2007, 17, 457; Rowe et al., Chem. Mater. 2004, 16, 3513; Murphy et al., J. Phys. Chem. B 2005, 109, 13857). These changes affect the signal features, the chemical selectivity, the morphology, the baseline resistance, and/or the stability and performance over time of the MCNP chemiresistive films (Wang, et al., J. Mater. Chem. 2007, 17, 457; Garg et al., Nanotechnology 2010, 21, 405501; Wang et al., Langmuir 2010, 26, 618; Nath & Chilkoti, in Engineering in Medicine and Biology, 2002. 24th Annual Conference and the Annual Fall Meeting of the Biomedical Engineering Society EMBS/BMES Conference, 2002, Proceedings of the Second Joint, Vol. 1, 2002, pp. 574-575; Rowe et al., Chem. Mater. 2004, 16, 3513; Joseph et al., J. Phys. Chem. C. 2007, 111, 12855; Cooper et al., J. Anal. Chem. 2010, 82, 3788; Joseph et al., J. Phys. Chem. B 2003, 107, 7406). As an illustrative example, Han et. al. (Anal. Chem. 2001, 73, 4441) have shown that a change in the NP size from 5 nm to 2 nm could affect the sensitivity of a specific MCNP chemistry, under similar exposure conditions to analytes, by ca. 35%. Dasog et al. (Langmuir 2007, 23, 3381) have shown that changes in the core size affect the oxidation rate of the organic ligands adsorbed on the NP surface, causing different drift in the sensing signal.
Joseph et al. (J. Phys. Chem. C 21108, 112, 12507) showed that, for individual MCNP-dominated morphology, the MCNP film is not conductive and the chemiresistor device shows no response. For island-dominated morphology, charge transport becomes possible after a ID percolation pathway is formed. This percolation pathway contains large island-to-island gaps, which are the bottlenecks for charge transport due to their high resistance. Changes in permittivity (Haick, J. Phys. D 2007, 40, 7173; Joseph et al., J. Phys. Chem. B 2003, 107, 7406; Steinecker et al., Anal. Chem. 2007, 79, 4977) and swelling-induced reduction in the island-to-island gaps decrease the resistance of MCNP films. For continuous 3D morphology, where many percolation pathways exist, the MCNP film swells along the direction perpendicular to the surface after dosage with analytes. As a result, the interparticle distances along these percolation pathways increase, and accordingly, the resistance of the MCNP film increases. Likewise, a decrease (shrinking) in the interparticle separation within the continuous 3D morphology usually leads to a decrease in resistance. The effect of the dielectric constant of the analyte on the sensing signal is presumably weaker, thus reducing the magnitude of the positive response for some of the analytes. This explanation is based on the assumption that the sensing mechanism remains the same (swelling/shrinking and dielectric permittivity changes) when percolation pathways have already been formed. Despite advances in this field, many of the underlying molecular mechanisms generating the sensing signal remain only vaguely understood (Haick, J. Phys. D 2007, 40, 7173; Kane et al., J. Mater. Chem. 2011, 21, 16846; Zabet-Khosousi & Dhirani, Chem. Rev. 2008, 108, 4072; Shuster et al., J. Phys. Chem. Lett. 2011, 2, 1912).
WO 2009/066293, WO 2009/118739, WO 2010/079490, WO 2011/148371, WO 2012/023138, US 2012/0245434, and US 2012/0245854 to some of the inventors of the present invention disclose apparatuses based on nanoparticle conductive cores capped with an organic coating for detecting volatile and non-volatile compounds, particularly for cancer diagnosis.
A problem often encountered in the diagnosis of diseases through the analysis of volatile organic compounds (VOCs) in breath samples is the sensitivity of sensing apparatuses to humidity. Since breath samples may contain up to 80% relative humidity (RH), the VOCs are often masked by water vapor which consequently impedes sensor performance.
Han et al. (Chem. Phys. Lett. 2002, 355, 405) studied the effects of relative humidity on the conductance of the assembly of poly(dG)-poly(dC) and poly(dA)-poly(dT) DNA molecules. The results show that the conductance of a specimen consisting of multiple DNA molecules might be strongly affected by the relative humidity. A similar effect was reported for silica gel surfaces (Anderson & Parks, J. Phys. Chem. 1968, 72, 3662). Guo et al., (Guo et al., Sens. Actuat. B 2007, 120, 521) showed that when alkanethiol-capped AuNP films that contained traces of the phase-transfer reagent tetraoctylammonium bromide (TOABr) were used as chemiresistive sensors, the film resistance decreased when the sensors were exposed to water vapor.
In order to overcome the effect of water vapor on sensor performance, a sensing apparatus is typically equipped with a humidity sensor (Yell & Tseng, J. Mat. Sci. 1989, 24, 2739) that independently measures the content of water vapor to be subtracted from the sensing signal thus affording the extraction of VOCs' signal.
There remains an unmet need of a sensing apparatus for detecting mixtures of VOCs in breath samples without dehumidifying the sample prior to measurement. There further remains a need for a humidity sensor having fast and reversible response upon exposure to water vapor.